This invention relates to superconductor power transmission lines and, more particularly, to the construction of a transmission line from superconductive elements having a crystallographic basal plane and anisotropic magnetic parameters relative to the basal plane, and wherein the superconductive elements are oriented with their basal planes perpendicular to the magnetic field of an electric current flowing through the elements of the transmission line so as to maximize the amount of current flow before introduction of magnetic hysteresis losses.
Superconductive material is employed currently in electrical equipment wherein it is essential to have large current densities. An example of such electrical equipment is nuclear magnetic resonance imaging equipment wherein inductor coils carrying relatively large amounts of direct current are employed for producing the relatively large magnetic fields required for activating the molecular electron energy levels in the detection of the emissions of radiation for forming the image. The flow of large currents in the conductor coils is made possible by a construction of the coils by superconductive material which operates without electrical resistance and without the generation of heat which would limit the current to much smaller values as is in the case of conventional conductor coils of the same physical size. As is well known, the superconductive material employed in present electrical equipment exhibits the superconductive characteristic only at very low temperatures, such as that of liquid helium. The low temperature requirement presents an inconvenience and additional cost in the operation of the equipment because of the need to pump liquid helium through the conductor coils to maintain the requisite low-temperature operating environment for the superconductor material.
Recently, significant amounts of research have been undertaken for the involvement of superconductive material which can operate at significantly warmer temperatures than the temperature of liquid helium. The availability of such materials suggest that it may be possible to construct electrical equipment of superconductive elements wherein the cooling requirement is less severe. This would reduce the task of cooling the superconductive material. For example, if superconductive material operative at much warmer temperatures such as the temperature of liquid nitrogen, rather than the temperature of liquid helium, would be available for the construction of electrical equipment, the cost of operating the equipment would be greatly reduced because of the greater availability of liquid nitrogen as compared to the availability of liquid helium.
A number of materials exhibiting superconductivity at temperatures elevated with respect to that of liquid helium have been investigated. Noteworthy among these materials are mixed metallic oxides which may be characterized, generally speaking, as ceramics. The following literature is exemplary of published articles describing the manufacturer and characteristics of materials having semiconductor characteristics at elevated temperatures. The Japanese newspaper SAHI SHINBUN issue of Mar. 10, 1987, discloses a compound of La, Cu, Sr, and O which has been formed as a ceramic tape with a thickness of 20-30 microns. The Japanese JOURNAL of APPLIED PHYSICS, Vol. 26, No. 4, April 1987, pages L386-L387 discloses a Ba-La-Cu-O and a Sr-La-Cu-O system having phases which undergo a superconducting transition. The report on the meeting of the Materials Research Society, April 1987, presents a report by S. Jin et al disclosing Ba.sub.2 YCu.sub.3 O.sub.7 alone and as a composite with silver at pages 219-221. The PHYSICAL REVIEW LETTERS, Vol. 58, No. 25, June 1987, pages 2688-2689 discloses inhomogeneities and magnetization hysteresis with respect to Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x. The Japanese JOURNAL OF APPLIED PHYSICS, Vol. 26, No. 4, April 1987, pages L524-L525, discloses superconducting single crystal thin films described by the formula (La.sub.1-x Sx).sub.2 CuO.sub.4 with respect to the Hall effect in superconducting thin films The PHYSICAL REVIEW LETTERS, Vol. 58, No. 25, June 1987, in an article on pages 2687-23690 by T. R. Dinger et al discloses single crystals of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x having facets measuring 200 microns and larger up to 0.5 millimeters, and having anisotropic characteristics in a layered structure. An article in The PHYSICAL REVIEW LETTERS, Vol. 59, No. 10, September 1987, pages 1160-1163 by T. K. Worthington et al discloses critical magnetic fields in different directions in a single crystal of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x. An article in The PHYSICAL REVIEW, Vol. 36, No. 7, September 1987, pages 4025-4027 by D. E. Farrell discusses alignment under magnetic field of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x. An article in SCIENCE, Jun. 5, 1987, page 1189 discloses the production of superconductors as epitaxial layers or films on substrates of strontium titanate. An overview of superconducting materials such as the Y-Ba-Cu-O system and related substances is disclosed in a book entitled NOVEL SUPERCONDUCTIVITY by S. A. Wolf and V. Z. Kresin published in 1987 by Plenum Press, New York, pages 935-949. An article in APPLIED PHYSICS LETTERS, Vol. 54, No. 11, March 1989, pages 1054-1056 by G. Koren et al discloses superconducting material fabricated as epitaxial films of YBa.sub.2 Cu.sub.3 O.sub.7-x on substrates of NdGaO.sub.3, LaGaO.sub.3, and SrTiO.sub.3 by Laser ablation.
One significant aspect of the foregoing studies is that these materials are anisotropic, and that a magnetic field, whether generated by a current flowing within the material itself or generated by an external current can terminate the superconductive phase. Also of significance is the fact that magnetization hysteresis associated with alternating current flow may introduce losses and heating associated with such losses. Such hysteresis does not present a limitation on the superconductivity for electrical equipment employing direct current (DC), but would present a limitation by way of example, in the important situation of transmission of power by alternating current (AC). The directions of the magnetic field are given with respect to the basal crystallographic plane useful in describing the anisotropic properties of superconductive materials such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x wherein the plane is defined by an oxide of copper. The magnetic fields, or components thereof, are either parallel to or perpendicular to the basal plane.
A problem arises in that there are limitations on the use of such superconductive materials for the transmission of AC power. One limitation on the maximum current that can be efficiently transmitted is the introduction of losses by magnetization hysteresis. A second limitation is the quenching of the superconductivity by excessively large magnetic fields produced within an electrical conductor by current flowing through the conductor, or through a second conductor lying alongside the first conductor as in the case of a transmission line comprising a pair of conductors or a plurality of conductors as in three-phase or multi-phase power transmission. Existing transmission line structures are not configured for maximizing the current and power transmission capabilities of superconductive materials.